This invention relates to a monochrome or color filter array element. More particularly this invention relates to display devices incorporating the monochrome or color filter array element such as liquid crystal displays.
Thermal transfer processes that selectively impinge radiation on a layer to transfer a material such as a colorant to a receiver are known. Lasers are useful sources of such radiation, particularly infrared lasers, which are readily available, easily used, and powerful. Thermal transfer processes are imaging processes used in applications such as color proofing, electronic circuit manufacture, monochrome and color filters, and lithography.
Monochrome and color filters are useful for liquid crystal display devices such as flat panel display devices. Liquid crystal display devices are known for digital display in electronic calculators, clocks, household appliances, audio and visual equipment, etc.
Radiation filters, monochrome filters, color filters, flat panel displays and liquid crystal displays are described in “Fundamentals of Active-Matrix Liquid-Crystal Displays”, Sang Soo Kim, Society for Information Display Short Course, 2001, and in a series of publications from the Materials Research Society, located in Warrendale, Pa. (e.g. volume 345, titled “Flat Panel Display Materials”, edited by J. Batey, A. Chiang, and P. H. Holloway, ISBN 1-55899-245-6; volume 424, titled “Flat-Panel Display Materials II”, edited by Miltiadis K. Hatalis, Jerzy Kanicki, Christopher J. Summers, and Fumiaki Funada, ISBN1-55899-327-4; volume 471, titled “Flat Panel Display Materials III”, edited by R. Fulks, G. Parsons, D. Slobodin, and T. Yuzuriha, ISBN 1-55899-375-4; and volume 508, titled “Flat-Panel Display Materials 1998”, edited by G. Parsons, T. S. Fahlen, S. Morozumi, C. Seager, and C-C. Tsai, ISBN 1-55899-414-9).
Certain thermal transfer processes are known. These known processes include dye sublimation, dye transfer, melt transfer, and ablative material transfer. These processes typically use an assemblage comprising (a) a donor that contains a transferable layer with a material to be transferred (for example a colorant such as a dye or a pigment) and a support for the transferable layer, and (b) a receiver, the receiver having a receiving surface closely aligned and in at least partial contact with the material to be transferred. The assemblage is imagewise exposed by radiation, typically infrared radiation from an infrared laser scanning over portions of the assemblage directed towards selected regions of transferable material, resulting in a selective transfer of the selected regions of material from the transferable layer of the donor to the receiver, on or through its receiving surface. Typically a mask is not necessary since only a small part is irradiated at a time. Each imagewise-controlled exposure typically takes place in a small, selected part of the assemblage at one time, so that the transfer of the material from the imageable element to the receiver element can be built up one region at a time. Exposure can be continuous or intermittent over the total time and area necessary. When exposure is complete, the built-up regions from the exposed parts can be contiguous or separate. Computer control can accomplish the exposure and transfer with high resolution and at high speed. The assemblage, after the imagewise exposure to the radiation from the laser as described supra, is henceforth termed an exposed assemblage.
The exposed assemblage can be separated into two elements, a spent donor retaining material from unexposed regions and the support, and an imaged receiver containing the receiver and the transferred material from exposed regions. Transfer of material to the receiver by exposure need not be complete and total over the exposed regions after separation in order to be useful; some material from exposed regions may be retained on the spent donor. Retention of material in the unexposed regions on the spent donor need not be complete in order to be useful; some may be transferred to the imaged receiver. However, differentiation of the amount of material transferred between exposed and unexposed regions is necessary. Nearly complete transfer by exposure and nearly complete retention without exposure is typical for mass transfer methods. Incomplete transfer by exposure is typically achieved with dye sublimation methods.
In laser induced thermal imaging processes, radiation absorbers are typically a component of the donor, so as to effectively convert the scanned radiation into heat, which initiates the thermal process responsible for transfer. Examples of radiation absorbers are thin metal layers, radiation-absorbing pigments, and/or radiation-absorbing dyes. Infrared-absorbing dyes are a specific example useful with infrared lasers as the radiation source for thermal imaging. In some cases, a pigment or dye used as a colorant simultaneously serves adequately as a radiation absorber, but in many cases a supplemental radiation absorber is necessary to convert enough of the scanned radiation into heat in order to transfer material.
Although a radiation absorber may be a necessary component of a donor, its use can introduce well-recognized problems. Opaque or colored radiation absorbers may interfere with achieving a desired transparency or color. Radiation absorbers may be transferred inconsistently with different exposure conditions, giving variable results of transparency or color unsuitable for demanding applications. Radiation absorbers of initially low color and opacity can be changed by exposure to give opaque or colored products, possibly changing over a period of time such as hours or days.
The presence of the infrared-absorbing dye among the materials transferred can be an undesirable but inevitable occurrence for many choices of infrared-absorbing dye, transferable material, assemblage, and exposure conditions. Although the infrared-absorbing dye is known to be capable of improving the production of the imaged receiver by absorbing radiation during the exposure step and facilitating an image-wise transfer, the transfer of infrared-absorbing dye during exposure is typically undesired. Infrared-absorbing dye transferred onto the receiver can produce a first problem of undesirably adding color to the receiver. The transferred infrared-absorbing dye may also be less stable than other color contributors such as pigments, producing a second problem of a color change over a period of time. The color contribution and change in color contribution of transferred infrared-absorbing dye can be dependent on many factors such as variable exposure conditions and positioning on the receiver, leading to irreproducibility of color in exposed receivers. However, typically many hundreds or thousands of receivers must be manufactured to meet a specific color specification.
Bleaching agents for thermal transfer processes are known. Certain bleaching agents have been combined with heating or exposure to light, to improve the utility of thermal imaging applications processes, and products by eliminating the presence of the infrared-absorbing dye after the imaging step.
Using a chemical bleaching agent and optionally a processing step such as additional radiation exposure and heating to remove the color contribution of transferred infrared-absorbing dye can be an undesirable solution to the foregoing problems. The introduction of chemical bleaching agents can be a complex process with unintended side-effects, such as physical damage, chemical damage to materials other than the infrared-absorbing dye, regulatory and health concerns, expense, shelf life and distribution channel complexities, and other known concerns.
There is a well-recognized need to improve the utility of radiation absorbers used in thermal transfer processes by minimizing their contributions to color and color variation after exposure. Placing the radiation absorber in a separate layer that is not transferred with colorant can minimize variable amounts of transfer; however, this separation may affect the utility of the radiation absorber. Special radiation absorbers can be provided which decompose upon the heating provided by laser exposure; however such special radiation absorbers may have low shelf life.
Colored radiation absorbers and radiation absorbers that change in color after exposure can be bleached by chemical bleaching agents added to the donor or receiver, or introduced at a later time. The bleaching speed or completeness of chemical bleaching agents may be improved by further exposure to radiation or heating. However, chemical bleaching agents introduced to the donor or receiver can lower the shelf life of the donor, receiver, or assemblage. Introducing chemical bleaching agents after exposure can be inconvenient, expensive, or can physically damage a transferred layer.
There remains a need for improved and simplified thermal imaging processes that can utilize a variety of infrared-absorbing dyes to produce objects of a specific color free of unwanted color attributable to the infrared-absorbing dye color.